In apparatuses that use hydrogen for a process of washing a semiconductor, apparatuses, such as medical or welding equipment, that use oxygen, and fuel apparatuses that use hydrogen and oxygen together, it is required that the dew point of water vapor contained in the hydrogen or oxygen be a value suitable for the respective purposes.
Therefore, in general, the dew point of water vapor in the supplied hydrogen or oxygen is adjusted to a very low degree, for example, about −40° C., and then, the dew point of water vapor is adjusted by humidification in a preceding step in each apparatus that uses hydrogen or oxygen.
Accordingly, in conventional hydrogen/oxygen-producing apparatuses that produce and collect hydrogen and oxygen through electrolysis of water, it is required that, after the water or water vapor contained in the produced hydrogen or oxygen is once removed, the dew point of water vapor be lowered. Therefore, the apparatuses generally include a gas-liquid separation device and a dehumidifier.
A conventional hydrogen/oxygen-producing apparatus 81 will be described below with reference to a schematic diagram of FIG. 9. The conventional hydrogen/oxygen-producing apparatus 81 includes a pure water-producing device 1 that produces pure-water, which serves as a material for gaseous hydrogen or oxygen; pure water tanks 2a and 2b at the hydrogen-production side and the oxygen-production side, respectively, for storing pure water; an electrolysis cell 3 that produces gaseous hydrogen and oxygen through electrolysis of the pure water; gas-liquid separation devices 4a and 4b at the hydrogen-production side and the oxygen-production side, respectively, that separate the gaseous hydrogen and oxygen, respectively, from the pure water, which is a liquid; dehumidifiers 5a and 5b at the hydrogen-production side and oxygen-production side, respectively, that remove water droplets and water vapor, which is a gas, from the gaseous hydrogen or oxygen; and storage tanks 6a and 6b at the hydrogen side and the oxygen side, respectively, for storing the gaseous hydrogen and oxygen, respectively, from which water and water vapor have been removed.
In the conventional hydrogen/oxygen-producing apparatus 81 of FIG. 9, before hydrogen and oxygen are produced, a preparation in which flow channels present at the hydrogen-production side and the oxygen-production side are filled with pure water in a flowing state (a flowing state refers to a state in which water circulates) is required.
That is, at the hydrogen-production side, the hydrogen-production-side flow channel of the electrolysis cell 3 and the lower part of the gas-liquid separation device 4a at the hydrogen-production side are filled with pure water from the hydrogen-production-side pure water tank 2a by a pump 12a. In the same manner, at the oxygen-production side, the oxygen-production-side flow channel of the electrolysis cell 3 and the lower part of the gas-liquid separation device 4b at the oxygen-production side are filled with pure water from the oxygen-production-side pure water tank 2b by a pump 12b. 
In addition, water levels of the pure water tanks 2a and 2b are maintained between upper and lower limits by means of supply pumps 10a and 10b and valves 9a and 9b or drain valves 11a and 11b, respectively, that are controlled by water level sensors 7a and 7b and controllers 8a and 8b. 
In the same manner, water levels of the gas-liquid separation devices 4a and 4b are also maintained between upper and lower limits by means of valves 9c and 9d that are controlled by water level sensors 7c and 7d and controllers 8c and 8d. 
When, in this manner, the hydrogen-production-side and oxygen-production-side flow channels of the electrolysis cell 3 are filled with pure water in a flowing state, and then, a direct-current voltage is applied between positive and negative electrodes of the electrolysis cell 3 by means of a power-supply device (not shown in the figure), hydrogen produces from the negative-electrode-side flow channel, while oxygen produces from the positive-electrode-side flow channel, and the produced hydrogen and oxygen penetrate into the gas-liquid separation devices 4a and 4b, respectively, together with pure water. Liquid portions of pure water accumulate in lower parts of the gas-liquid separation devices 4a and 4b, and gaseous portions such as hydrogen and oxygen permeate the upper part of the devices. Then, when the hydrogen and the oxygen pass through the dehumidifiers 5a and 5b at the hydrogen-production side and the oxygen-production side, respectively, water and gaseous water vapor are removed from them, and the hydrogen and the oxygen are stored in the storage tanks 6a and 6b at the hydrogen-production side and the oxygen-production side, respectively. However, there is the following problem in the above-described conventional hydrogen/oxygen-producing apparatus 81.
In the above-described conventional hydrogen/oxygen-producing apparatus 81, as amounts of water that are removed from the hydrogen and the oxygen by means of the humidifiers 5a and 5b approach an upper limit of dehumidification by the dehumidifiers 5a and 5b, dehumidifying performance of the dehumidifiers 5a and 5b will be deteriorated.
Therefore, a restoration in which water is released from the dehumidifiers 5a and 5b to the outside is required. For the restoration, for example, it is required that, when the dehumidifying performance is deteriorated, applying a voltage to the electrolysis cell 3 is discontinued, and the dehumidifiers 5a and 5b are replaced with other unused dehumidifiers.
Alternatively, for example, a structure that can realize the following operation would be required. That is, multiple dehumidifiers 5a and 5b are respectively connected with each other, and, by action of valves, they are each brought into a state in which only one of them is used, and they are each brought into another state in which one of the other unused dehumidifiers is used when the amount of removed water approaches the upper limit of dehumidification.
Thus, in order to maintain dehumidifying performance of the dehumidifiers 5a and 5b, such large-scale equipment is required. Alternatively, a discontinuation period for exchanging devices is required upon the operation. As a result, there is a limit to improving the operational rate of the apparatus.
Hence, in order to make it unnecessary to utilize the dehumidifiers 5a and 5b, a hydrogen/oxygen-producing apparatus 82 in which, by arranging gas-liquid separation devices 4a and 4b as cooling types, gases and liquids are separated from each other inside the gas-liquid separation devices 4a and 4b, and a dew point of water vapor is lowered has been proposed (see Japanese Patent No. 333555).
The hydrogen/oxygen-producing apparatus 82 of Japanese Patent No. 333555 will be described based on the schematic diagram of FIG. 10. Matters not mentioned herein are the same as FIG. 9.
With regard to the hydrogen/oxygen-producing apparatus 82 of FIG. 10, its configuration downs treats of the electrolysis cell 3 (an upper part of the figure) differs from the hydrogen/oxygen-producing apparatus 81 of FIG. 9. That is, the differences relate to structures of the gas-liquid separation devices 17a and 17b, and a feature that dehumidifiers 5a and 5b as found in FIG. 9 are not provided downstream of the gas-liquid separation devices 17a and 17b, and the gas-liquid separation devices 17a and 17b are connected directly to storage tanks 6a and 6b. 
Hydrogen and oxygen produced in the electrolysis cell 3 are delivered into pure water that has permeated first chamber 13a and 13b of the gas-liquid separation devices 17a and 17b, and move upward inside the pure water, and, consequently, gases are separated therefrom. Since the pure water is maintained at a low temperature by means of cooling devices 15a and 15b, a dew point of water vapor inside the hydrogen and oxygen is lowered while the hydrogen and oxygen move upward inside the pure water. Furthermore, cooling devices 16a and 16b are provided in spaces above the first chambers 13a and 13b, respectively. The hydrogen and oxygen that pass therethrough are cooled by the cooling devices 16a and 16b. Accordingly, the dew point of water vapor decreases. The hydrogen and the oxygen in such a state are stored in storage tanks 6a and 6b. In addition, when the water levels of the first chambers 13a and 13b are full, the pure water moves across partition walls α and β, enters second chambers 14a and 14b, and is further delivered back to the pure water tanks 2a and 2b. According to the hydrogen/oxygen-producing apparatus 82 of Japanese Patent No. 333555, it is not required that dehumidifiers 5a and 5b for decreasing a dew point of water vapor contained in the hydrogen and oxygen be provided therein, problems of an increase in size and complexity of the equipment can be overcome, and it is not required that any discontinuation period for exchanging devices upon the operation be set.